1. Field of the Invention
The present invention relates to communication systems, and, in particular, to mobile telephony systems that conform, for example, to the IS-95 standard.
2. Description of the Related Art
FIG. 1 shows a block diagram of part of a mobile telephony system conforming to the IS-95 standard. In the forward link, signals from base station 102 are transmitted by base station antenna 106 to mobile unit 104. Analogously, in the reverse link, signals from mobile unit 104 are received at base station antenna 106 for base station 102. Antenna 106 is typically hard-wired to base station 102 and signals are transmitted between base station 102 and antenna 106 over that hard-wire connection 108.
The timing of the transmission of forward-link signals within an IS-95 communication system is based on a global positioning system (GPS) time reference (called "GPS time 0") derived at the base station, using a GPS receiver (not shown). Each mobile unit derives its own time reference based on the earliest arriving multi-path component of the forward-link signals transmitted by the base station. Each mobile unit uses its derived time reference to control the timing of the transmission of reverse-link signals.
In an IS-95 communication system, there will be a forward-link delay between the time at which forward-link signals are time-stamped by a base station and the time at which the signals are actually transmitted by each of its antennas. These forward-link delays are due, at least in part, to the physical separations between the base station and its antennas, such as that shown in FIG. 1, which separations may vary from antenna to antenna and from base station to base station in a communication system. Processing delay (e.g., from D/A conversion and modulation) also contributes to the duration between time-stamping and actual transmission.
According to the IS-95 specification, a base station advances the timing of the forward-link signals by an amount (called the transmit delay) corresponding to the smallest forward-link delay between time-stamping at the base station and actual transmission by one of the base station antennas, so that the actual transmission of signals by that base station antenna will begin at GPS time 0. The magnitude of this transmit delay is based on empirical measurements made when the base station and antenna are first configured. For base stations that have two or more different antennas, the transmit delay is based on the antenna having the smallest forward-link delay. Each mobile unit will derive its own time reference based on the earliest arriving multi-path component of the forward-link signals transmitted by the base station and use the derived time reference to transmit reverse-link signals back to the base station.
The base station defines an access-channel search window within which the base station scans for reverse-link signals transmitted by the mobile unit in the access channel. In conventional IS-95 systems, the base station access-channel search window may begin, but does not have to begin, at GPS time 0.
Referring again to FIG. 1, as in the forward link, for each base station antenna, there will be a reverse-link delay between the time at which reverse-link signals are received at each base station antenna (e.g., 106) and the time at which the received reverse-link signals are processed by base station 102. This reverse-link delay is due at least in part to the time that it takes to transmit the received signals from antenna 106 to base station 102 over hard-wire connection 108. In general, for base stations with more than one antenna, the smallest reverse-link delay is referred to as the receive delay. The existence of these reverse-link delays means that no signals received from a mobile unit, such as mobile unit 104, will ever be present at the base station before the expiration of the receive delay. This will cause the arrival range of the reverse signal to be shifted by the receive delay. As a result, the upper limit of the base station access-channel search window is also shifted by the receive delay.
The upper limit of the base station access-channel search window is related to the virtual cell size and the receive delay between antenna 106 and base station 102. Based on hardware constraints and the pilot PN code reuse factor, the upper limit has to be less than a specified value. The virtual cell size is decided by the maximum differential delay (i.e., the maximum difference in reverse-link delay between any two antenna connectors in the same base station sector), the probe randomization delay (i.e., a delay that the mobile unit waits before transmitting a probe in the access channel), and the cell size (i.e., the radius of the coverage area for the antenna connector with the largest coverage area in a particular sector). These three values that are used to determine the virtual cell size are typically large. The value of the virtual cell size is confined by the upper limit of the base station access-channel search window minus the receive delay. This constraint is referred to as the delay budget. Thus, the larger the receive delay, the tighter the delay budget. Furthermore, the tighter the limit on the virtual cell size, the harder it is to configure the base station to stay within the delay budget.